Dittel TMP608

Walter Dittel GmbH was founded in 1959 in Landsberg, Germany and started manufacturing of aircraft radios and other products for avionics. Dittel introduced in Summer 1972 with the W200 one of the first battery-powered electronic calculators manufactured in West Germany. The W200 started a successful line of electronic calculators sold under various names, including MBO - think MBO Junior. Dittel stopped manufacturing calculators already in 1974 due to the rapid decline of retail prices for pocket calculators.

Here at the Datamath Calculator Museum we acquired this TMP 608 calculator in 2023 for its TMS0604 single-chip calculator circuit on our quest to complete the Characterization of Single-Chip Calculator Circuits of the TMS0600 Product Family.

Dismantling the featured Dittel TMP608 calculator manufactured in August 1974 in Germany requires the removal of four screws and is instantly rewarding with the very high manufacturing quality of Dittel products. Every part of the calculator feels very substantial and well engineered, from the latching battery cover to the printed circuit board (PCB) traces finished in gold. The Texas Instruments Klixon™ keyboard used with earlier designs of Dittel was replaced with snap action switches placed directly onto the PCB of the calculator, lowering the complexity and manufacturing costs of the calculator substantially.

Calculating Unit: The TMS0604 is a member of the TMS0600 Product Family and tracing back to the TMS1802NC, the first available standard calculator building block on a chip, later renamed into TMS0102. The TMS0600 increased the size of the Instruction ROM (Read-Only Memory), kept the Data Memory and added integrated segment drivers for the LED display.

Display: This Dittel TMP608 calculator manufactured in August 1974 makes use of three Litronix DL330 3-Digit display modules with three 7-Segment displays chips bonded onto a PCB and magnified with a clear plastic lens. The three DL330 modules are soldered directly onto the PCB of the calculator.

Display Driver: The PCB of the disassembled Dittel TMP608 makes use of two SN75492 Digit Drivers and utilizes the integrated Segment Drivers of the TMS0604 chip. We noticed two 10 Ohm resistors wired to the Digit Drivers, effectively limiting the maximum digit currents of the LED display.

Clock: The Dittel TMP608 uses the switching frequency of its DC/DC converter for the clock input pin of the TMS0604 single-chip calculator circuit, we measured with the featured calculator a clock frequency of about of 160 kHz.

Power Supply: The Dittel TMP608 is powered by four disposable AA-sized 1.5 Volt batteries and can be operated with an external, DC adapter, too. The PCB of the calculator hosts a discrete power converter to generate the V_GG supply voltages for the TMS0604 chip while the V_DD pin is connected directly to the batteries. We observed with the featured calculator manufactured in August 1974 voltages of V_DD = -6.0 V and V_GG = -12.7 V while operated with V_BAT = 6.0 V. We measured the operating current of the featured Dittel TMP608 calculator for two different cases:

The power consumption for the featured Dittel TMP608 results in about 280 mW displaying a '0.' and 900 mW with all segments but the minus sign illuminated. The peak current of 150 mA with all segments lit is unusual high and a result of the strong integrated Segment Drivers of the TMS0604 and the lack of additional segment current limiting resistors found with other designs. The Brother Model 827R (Version 2) calculator sporting a similar TMS0603 chip and hosting these extra resistors clocks in at 280 mW and 440 mW, respectively.

Keyboard: The Dittel TMP608 calculator integrates the keyboard contacts on the PCB, hence the gold finish on the PCB traces that protects the copper conductor from oxidation and corrosion. The 23 snap action discs are brought into position with a thin plastic sheet and hold in place with an adhesive film while the three sliding switches use small plastic chambers to hold the respective contacts.

Preparing our DCM-50A Platform to allow the Characterization of Single-Chip Calculator Circuits of the TMS0600 Product Family, we studied the featured Dittel TMP608 calculator manufactured in August 1974. In a first step did we observe with a Mixed Signal Oscilloscope (MSO) the signals at the TMS0604 chip to verify its pin-out before disassembling the calculator completely to analyze its printed circuit board (PCB) wiring.

Confirming the pin-out of the calculator chip and reverse-engineering the keyboard matrix, we were able to fully operate the TMS0604 with our DCM-50A Platform and not only measure precisely the timing of its Display and Keyboard interface but digging deeper into the algorithm embedded in its firmware, leading to an unexpected discovery:

Flag-Information Output: While reverse-engineering the electrical connections of the Memory Indicator LED of the Brother Model 827R (Version 2) calculator to its TMS0603 chip, we noticed that it is wired between the H-Segment of the TMS0603 and the common cathode of Digit 8 of the LED display.

The H-Segment Output of the TMS0600 device is a relict inherited from the TMS0100 Product Family to render the "Fancy Four" used with some early Vacuum Fluorescent Displays but not available with 7-Segment LED Displays like the one used with the Dittel TMP608 calculator. With the DCM-50A Platform using eleven H-Segment LEDs and most of them turning on and off at seemingly random times while operating the TMS0604 chip, we started to analyze each of the eleven signals during different sample calculations. To our surprise could we decipher the "Flag-Information" encoded in ten of the 11 Digit Times:

Analyzing the Feature Sets and Calculator Logic Implementations of the six known members of the (not so successful) TMS0600 Product Family, we can summarize: Too Little, Too Late. When Texas Instruments introduced the TMS0600 early in 1974, with custom-specific designs not available before Summer 1974, the market of pocket calculators diverted into two directions:

With the rather limited program memory (384 Words * 11 Bits) and small data memory (3 * 13 Digits Registers), the functionality of all six known TMS0600 designs wasn't competitive and the restriction to LED displays with external digit drivers and missing an internal clock oscillator didn't help with the manufacturing costs of an electronic calculator.

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, December 22, 2024. No reprints without written permission.